An official from US forces-Korea said that the nuclear-powered aircraft carrier USS George Washington had joined the four-day exercises.

North Korea responded by placing surface-to-surface missiles on launch pads in the Yellow Sea, the Yonhap news agency reported. It said the state had moved surface-to-air missiles near frontline areas.

North Korea has promised a "merciless military counterattack" against any intrusion into its territorial waters, the country’s official news agency reported.

Leonid Petrov, a lecturer in Korean Studies at the University of Sydney, told Al Jazeera that the military drill was "a rather risky undertaking," given the unresolved dispute over the waters. Politicians should be placing more emphasis on dialogue to cool the tensions, he argued.

"Of course rather than talking, negotiating, giving diplomats an opportunity to discuss the future development of co-operation and exchange, we see that the military is given the full reign in sorting out the issue," Petrov said.

Artillery fire?

South Korea's military later said that explosions - possibly the sound of artillery fire - were heard on Yeonpyeong Island on Sunday.

Residents, journalist and marine of South Korean take a sheltered in bunkers for 40 minutes [EPA]

South Korea's Joint Chiefs of Staff said that what is believed to have been a round of artillery was heard on Sunday from a North Korean military base north of the sea border dividing the two Koreas. It was not immediately clear where the alleged round landed.

Residents of the island were ordered to take shelter in underground bunkers, but that order was later withdrawn, according to Yonhap, a South Korean news agency.

Dozens of reporters, along with soldiers and police and a few residents, headed for the bunkers, where they remained for 40 minutes.

"The order was lifted when no more sounds were heard," a spokesperson for the South Korean defence ministry said.

Al Jazeera's Marga Ortigas, reporting from the South Korean capital, Seoul, said that military leaders had said that the military exercises were taking place "nowhere near the contested area" between North and South Korea in the Yellow Sea.

Games go south

Officials say the drill is now being held 120km south of the contested border. There is no explanation about the change in location.

"We don't know if this was some intentional change of location in some way to try to not send out signals of provocation to North Korea or if indeed they might say it was exactly where it was meant to be in the first place," she said.

Lee Myung-bak, the South Korean president, warned ministers and aides to be ready for further "provocation" by North Korea during the drill.

"There is the possibility that North Korea may do some unexpected action, so please perfectly prepare against it through cooperation with the Korea-US joint force," Lee was quoted by a spokesman as saying.

Nerves are also high on the South Korean mainland, where the population is being advised to seek shelter in the subway in the event of open conflict.

"The population of South Korea understands that things are getting tense," Petrov said.

'Ensuing consequences'

China has warned South Korea and the US that it "could not be responsible for what North Korea might do" if they went ahead with the military exercise, according to Al Jazeera’s correspondent.

"The Chinese have made it very very clear that they didn’t want these drills to occur at all," Ortigas said.

China sent senior officials including Dai Bingguo, the state councillor, to a meeting in Seoul on Sunday.

South Korea's Lee told the visiting officials that China must take a "more fair position" on the Korean Peninsula.

The Chinese delegation agreed with South Korea that the situation was "worrisome" and that it would try to stop it deteriorating, according to Chinese media.

The US military said the exercises, planned long before Tuesday's attack, were designed to deter North Korea and were not aimed at China.

"These latest provocations have been by the North and they need to take ownership of those, not us."

But Petrov told Al Jazeera that attempting influence China to change its stance on its ally is a dangerous game to play.

"It is pointless to pressure China to put more pressure on North Korea," Petrov said.

"The decision of the United States to send the [USS] George Washington nuclear carrier might really thwart the opportunity for President Hu Jintao to visit Washington in January and that would be a really bad development."

He said that Washington and Beijing were in a position to find a solution to the issue, if a more co-operative and less interventionist approach was taken.

"Wouldn't it be better to give the opportunity to the Korean Peninsula to stay nonaligned, independent, nuclear free?" he asked.

A top North Korean official is scheduled to visit China on Tuesday, the Xinhua news agency reported.

Choe Thae-Bok, the chairman of North Korea's Supreme People's Assembly and a close confidant of Kim Jong-il, the country's leader, was invited by Wu Bangguo, China’s second most powerful official, the report said.